x/y=3,则x=3y,于是(2x²-3xy+5y²)/(x²+3xy-y²）=(2*9y²-3y*3y+5y²)/(9y²+3y*3y-y²）=(18y²-9y²+5y²)/(9y²+9y²-y²）=(14y²)/(17y²...